Fat provides an engery store and can be used as a respiratory substrate when carbohydrate levels are low.
It is first broken up into glycerol and its fatty acids, by hydrolysis (using lipase enzyme). First glycerol is converted into a 3-carbon sugar, and feeds into glycolysis. The fatty acid chains are split into 2-carbon fragments which enter the Krebs cycle as acetyl Coenzyme A.
Fatty acid chains yeild large amounts of ATP (dependant on the length of the hydrocarbon chain), this is as many hydrogen ions are picked up by carriers (NAD and FAD) and are fed into the electron transport systems.
One gram of fat yeilds more than twice as much energy as one gram of carbohydrate.
Protein is only used as a respiratory substrate when an individual is suffering from starvation, when all carbohydrate and lipid stores have been used up.
Tissue protein becomes mobilised to allow it to be used for energy supply. Protein is hydrolysed (by protease) into its amino acids.
The amino acids are the deaminated in the liver, the amino group is converted into urea and is excreted. The residue is converted to a Krebs cycle intermediate (pyruvate, acetyl CoA etc.).
Acetyl CoA is like a crossroads in metabolism, it is formed during oxidation of carbohydrates, lipids and proteins, allowing all substrates to be fed into the krebs cycle at the same point.